This section is intended to provide information relevant to understanding various technologies described herein. As the section's title implies, this is a discussion of related art that should in no way imply that it is prior art. Generally, related art may or may not be considered prior art. It should therefore be understood that any statement in this section should be read in this light, and not as any admission of prior art.
Integrated circuits include circuitry to perform monitoring functions to assist with detecting whether a power supply has reached low power levels. Sometimes, integrated circuits are subject to insufficient power availability, and protection may be achieved with a type of circuit known as a brown-out detector (BOD).
FIG. 1 shows an example of a BOD circuit. In this circuit, first and second reference voltages are compared with an analog comparator circuit. The first reference voltage Vtr is a BOD trigger voltage that is set by supply voltage VDD and a ratio of resistors R1 and R2. The second reference voltage Vz is a Zener diode voltage that is independent of the supply voltage VDD and is set using an external Zener diode. Another resistor R3 is used to provide bias current to the Zener diode. As VDD goes low, the trigger voltage Vtr decreases based on a ratio of R1 and R2. When the trigger voltage Vtr becomes equal to or less then the Zener diode voltage Vz, the BOD circuit output goes high to indicate an error condition has occurred due to insufficient supply voltage. This error signal is given to a system power management controller to take action and save contents of memory and registers to main memory.
Unfortunately, some problems with conventional BOD circuits include situations where reference voltages that are used for comparison to monitor the supply voltage VDD can consume high DC current and continuously draw current from the supply. This current can be several hundred micro-amps (μA) to several milli-amps (mA). For example, some conventional BOD circuits consume currents within a 100 μA to over 1 mA range. Another problem with conventional BOD circuits is that, as supply voltages continue to scale, the Zener diode reference becomes more difficult to scale.